It is the same reason that in a human family with a blue eyed parent and a brown eyed parent, some of the biological kids may have blue eyes, some have brown eyes and some have something completely different from either parent! Or two brown eyed parents have a blue eyed kid!
Let’s take tomatoes, for instance, if you planted only Sungold tomatoes, which are a hybrid (F1), then saved the seeds, the seeds you gather from those plants (for next year) will produce a multitude of different types of tomatoes, only a few which “might” actually resemble their parent.
Other situations can also cause your “2nd generation” of plants to NOT resemble or taste like the parent.
If you planted two “varieties” of the same “species” near enough to each other that they cross pollinated, their offspring will be some mix of the different parents and not resemble the parents either. Or, if your nearest neighbors planted yet other varieties of the same species, your 2nd generation of plants might have cross-bred with those, as well!
Needless to say, farmers and seed sellers spend a lot of energy isolating and growing their plants carefully so that when you buy that seed packet or plant you will reliably get what you expect!
This shouldn’t stop you from experimenting or saving seeds, but, you might wish to do more research on what types of seeds or plants you are purchasing initially, and whether they are hybrids, open pollinated or the like. Then you won’t be surprised by what happens the 2nd, 3rd or even 10th year of saving seeds.
Some links you might wish to check out include:
Folksy discussion: https://www.motherearthnews.com/organic-gardening/save-hybrid-seeds-zbcz1602
Seed saving 101 from University of Minnesota Extension (PDF download): https://local.extension.umn.edu/sites/local-extension.umn.edu/files/download/Seed%20Saving.pdf
Highly technical discussions: https://www.growingformarket.com/articles/breeding-tomatoes-farm-practical-selection-advice
Or google: “saving tomato seeds and cross breeding”
Image: Community Garden Club of Camas Washougal